European fire ant helping spread of invasive plant

As if it wasn’t bad enough that our yards and parks are being taken over by a species of ants with a painful sting, now researchers say that these invasive insects are also helping the spread of an invasive plant species.

It is a double whammy, says Megan Frederickson, University of Toronto evolutionary biologist and one of the authors of the study to be published in the Proceedings of the Royal Society B Wednesday.

The results of the study of Myrmica rubra, the fire ant native to Europe that was first discovered in Ontario in 1970, suggest that “invasional meltdown could be happening right under our noses, here in Ontario,” said Frederickson.

“Invasional meltdown” is the idea that one invasive species could help the spread of another invasive species, making it more common in an eco-system than if it were invading a new place on its own.

“We don’t have a lot of examples,” said Frederickson. “But I think it is more common than we think it is.”

Research for the study was conducted at U of T’s field station, the Koffler Scientific Reserve, where the team created artificial ecological communities inside 42 small plastic children’s swimming pools. Each pool was filled with soil and planted with four species of spring wildflowers — three native species and one invasive.

Researchers then collected colonies of either the European fire ant or a native woodland ant and added them to the pools. The ants picked and moved seeds of these plant species as the researchers watched.

“The pools with the invasive ant were overrun by the invasive plant, but pools with the native ant had lots of native plants,” said Kirsten Prior, an ecologist and co-author of the paper.

The invasive ants moved lots of seeds of all four plant species but the invasive plant, greater celandine, took advantage of being dispersed more than the other species.

Greater celandine, the invasive plant species that the ants spread the most, is a weed commonly seen in Ontario that grows rapidly. It multiplies quickly in the same places where wildflowers like trilliums grow.

Traditionally people have studied invasive species in isolation, said Frederickson. “But it now seems that we have a lot of invasive species all arriving in new environments and they interact with each other. And those interaction help how fast they are able to take over.”

What is really worrying for the study’s authors is that if this kind of interaction between two invasive species is common, it could result in there being invasive species everywhere all the time and that invasive species could spread really fast.

How exactly the tiny reddish-brown fire ants — notorious for their painful, burning stings — travelled to North America isn’t clear but Frederickson says most invasive insects get to new places “because they are brought in with agriculture shipments . . . like crops. They are accidentally moved around the world.”

In Toronto, the ants, which seem to prefer moist areas, have become a lot more common in the past decade, say researchers. They are all over the Islands, parks like the Tommy Thompson Park, the Cherry Beach and the Don Valley ravine.

“Why it has taken this long for their populations to grow . . . isn’t clear,” said Frederickson.